Harvest may present some challenges when drought is a major factor. The last USDA-National Agricultural Statistics Service crop progress and condition report classified more than 40% of the soybean crop to be in good or excellent condition. Overall, 60% of all soybeans in Kansas are dropping leaves with a few areas of the state just entering into harvest (only 6% harvested).
The weather conditions expected for mid-September to mid-October will be critical for soybean as related to the seed filling and determining final seed weight.
While current conditions have been cooler-than-average, temperatures are expected to trend towards warmer-than-normal during the next 8 to 14 day period. This pattern is expected to continue for October. There is a slight chance for wetter-than-normal conditions in the short term (6-10 days), but the outlook for October favors drier-than-normal conditions in the eastern third of the state, and equal chances for the rest of the state. Note that there is a significant drop in normal rainfall amounts from September to October as we move into what is typically the drier part of the year.
Soybean will reach final maturity with high seed water content, moving from 90% to around 60% from seed filling until final maturity. Final maturity is defined as the formation of the black layer in the seeds. According to a recent Extension article from Iowa State, “How fast do soybeans dry down in the field?”, the average seed dry down rate was 3.2% per day. Still, the dry down rate will depend on the maturity group selection (affecting the length of the season), planting date, and weather conditions experienced during the latter part of the reproductive phase.
Changes in the water content during the seed-filling process (Figure 1) were previously described in our “Soybean Growth and Development” poster. As described for corn, seed water loss for soybeans can also divided in two phases: 1) before “black layer” or maturity, and 2) after black layer.
In order to properly address the question related to the dry down rate for soybeans, a study was conducted to investigate the changes in water content from black layer formation (maturity) until harvest time (Figure 2). During the last days of September and mid-October 2016, the overall dry down rate was around 3% per day (from 58% to 12% seed moisture) – taking an overall period of 15 days.
*Note: It is desired to reach harvest with 13% seed moisture to maximize the final seed volume to be sold, thus the importance of timing harvest with the right seed moisture content.
Soybean dry down rate was three-time faster, 3% per day, relative to corn at 1% per day. These dry down rates for corn and soybeans are primarily affected by temperature, humidity, and overall water content at the point of black layer formation (maturity). These main factors should be considered when the time comes to schedule soybean harvest.
For more information on dry down rates for corn, see the companion article, “Rate of dry down in corn before harvest”, located in this eUpdate issue.
This project is expected to be expanded in the coming years to include additional soybean producing regions and to consider other factors such as planting date, soybean maturity group, and diverse weather environments across the state. If you are interested in participating in this project, please contact the researchers listed below.
Ignacio A. Ciampitti, Crop Production and Cropping Systems Specialist
Santiago Tamagno, KSUCROPS Production, Dr. Ciampitti’s Lab
Damaris Hansel, KSUCROPS Production, Dr. Ciampitti’s Lab
Mary Knapp, Weather Data Library